The present invention relates to an electric power steering apparatus for an automobile, and more particularly, it relates to overheat protection of an electric motor for assisting a steering force.
A conventional electric power steering apparatus in which an electric motor is used as means for assisting a steering force has a configuration, for example, as is shown in a block diagram of FIG. 1. In this electric power steering apparatus, a torque sensor 1 provided on a steering shaft detects torque applied thereto, a vehicle speed sensor 2 detects a vehicle speed, and data detected by these sensors are input to a microprocessor 3. On the basis of the information supplied by the sensors 1 and 2, an assist characteristic deciding section 31 decides a current component for driving an electric motor 5 so as to cancel the detected torque. An inertia control characteristic deciding section 32 decides, also on the basis of the information supplied by the sensors 1 and 2, a current component for driving the electric motor 5 so as to cancel the inertia of the electric motor 5.
The value of the current component decided by the assist characteristic deciding section 31 is input to a protecting section 36, and the protecting section 36 decides an upper limit value and outputs the upper limit value to an adder 37. The value of the current component decided by the inertia control characteristic deciding section 32 is directly input to the adder 37, and the adder 37 obtains a sum of these values. The sum is then input to a subtracter 38.
A driving current for the electric motor 5 for assisting a steering force is detected by a current detecting element 6 and the thus detected value is supplied to the microprocessor 3. This data is also supplied to the subtracter 38 as feedback data and to the protecting section 36 to be used for determining the upper limit value.
A difference, that is, the output of the subtracter 38, is input to a motor driving circuit 4, so that the electric motor 5 can be driven by a PWM wave in accordance with the difference.
The upper limit value of the driving current for the electric motor 5 determined by the protecting section 36 is defined, for example, as is shown in FIG. 2. In FIG. 2, the abscissa indicates an integrated value of the driving current (overload protection integrated value), namely, (driving current).sup.2 /(1+Ts), wherein s indicates a Laplacean and T indicates a time constant depending upon a temperature increase characteristic of a transistor of the motor driving circuit. For example, T is equal to 16384 seconds. The ordinate indicates the upper limit value.
The integrated value indicates a value of primary delay corresponding to temperature increase caused by an exothermic amount expressed by a square of the current and the accompanied heat radiation amount, and simulates the temperature increase characteristic of the transistor of the motor driving circuit 4. As is obvious from FIG. 2, when the integrated value exceeds a predetermined value, (3.7).sup.2 /(1+Ts), the upper limit value is decreased from 60A, that is, a rated value.
Now, the operation of the protecting section 36 for determining the upper limit value of the driving current for the electric motor 5 will be described with reference to a flowchart for showing the operation shown in FIG. 3.
First, the protecting section 36 reads a current value detected by the current detecting element 6 (step S1), and calculates a square of the detected current value (step S3). Then, the square of this detected current value is added to an overload protection integrated value obtained in previous sampling, and the sum obtained through this addition is stored in a working RAM 1 (step S5).
Next, the protecting section 36 divides the content of the working RAM 1 by the time constant T, multiplies the obtained quotient by a sampling cycle (for example, 0.5 second), and stores the obtained product, as a value of primary delay, in a working RAM 2 (step S7).
Subsequently, the protecting section 36 subtracts the content of the working RAM 2 from the content of the working RAM 1, and defines the obtained difference as an overload protection integrated value (step S9). Then, on the basis of the thus obtained overload protection integrated value, the protecting section 36 determines the upper limit value of the motor current based on a table (not shown) listing the characteristic as is shown in FIG. 2 (step S11).
As described above, in the conventional electric power steering apparatus, the value of primary delay of the temperature increase in the motor driving circuit is obtained on the basis of a square of the motor driving current, and the upper limit of the driving current is determined on the basis of the obtained value of primary delay. Thus, the overheat protection of the motor driving circuit is performed. However, in the case where the time constant used for calculating the value of primary delay is set at a large value with priority given to sufficient overload protection, even when it is necessary to increase the upper limit of the driving current in response to the decrease of the motor driving current, the recovery increase of the upper limit value can be delayed. As a result, the driving current is suppressed for a long period of time, and the steering force is disadvantageously continuously insufficient.